1. Field
Example embodiments relate to a gate structure in a semiconductor device and a method of forming a gate structure in a semiconductor device. More particularly, example embodiments relate to a multi-layered gate structure and a method of forming the gate structure.
2. Description of Related Art
A conventional gate electrode is usually formed using polysilicon doped with impurities. As a semiconductor device is highly integrated, the gate electrode of polysilicon may not be properly employed in the semiconductor device because polysilicon has a relatively high resistance. Accordingly, a gate electrode having a polycide structure has been developed, which has a resistance lower than that of polysilicon. The conventional gate electrode having the polycide structure generally includes a polysilicon film and a metal silicide film formed on the polysilicon film. Here, a titanium silicide film or a tungsten silicide film may be usually used as the metal silicide film. However, the conventional gate electrode having the polycide structure may not meet the desired resistance of the semiconductor device according as the semiconductor device has an extremely high integration degree while ensuring adequate electrical characteristics of the semiconductor device.
Recently, a gate electrode has been developed to include a polysilicon film and a metal film provided on the polysilicon film to reduce the resistance of the gate electrode. Here, an ohmic film and a barrier film should be provided between the polysilicon film and the metal film because the metal film may be changed into a metal silicide film when the metal film is directly formed on the polysilicon film.
In the conventional gate electrode, the metal film, the barrier film and the ohmic film generally include tungsten, tungsten nitride and tungsten silicide, respectively. However, nitrogen included in the barrier film may be decomposed from tungsten nitride and combined with silicon in the ohmic layer during high temperature processes for forming the gate electrode. Thus, an irregular interface of silicon nitride may be generated between the barrier film and the ohmic film, to thereby considerably increase the resistance of the gate electrode.
FIG. 1 is a scanning electron microscope (SEM) picture illustrating the irregular interface caused by the agglomeration of silicon nitride in the conventional gate electrode. In FIG. 1, the conventional gate electrode includes a polysilicon film, a tungsten silicide film, a tungsten nitride film and a tungsten film sequentially stacked on a substrate.
Referring to FIG. 1, the irregular interface is easily generated between the tungsten nitride film and the tungsten silicide film during the high temperature processes for forming the conventional gate electrode because of the agglomeration of silicon nitride.
Considering the above-mentioned problems, titanium silicide or titanium has been used as the ohmic film in the gate electrode instead of tungsten silicide.
FIG. 2 is an SEM picture illustrating a relatively uniform interface in another conventional gate electrode having the ohmic film of titanium. In FIG. 2, the conventional gate electrode includes a titanium film, a tungsten nitride film and a tungsten film successively formed on a substrate.
As illustrated in FIG. 2, the interface generated between the tungsten nitride film and the titanium film is relatively uniform after performing high temperature processes. However, the gate electrode including the ohmic film of titanium has a sheet resistance larger than greater than about 80% compared with that of the gate electrode including the ohmic film of tungsten silicide because the grain sizes of tungsten in the tungsten film becomes small due to the titanium film when the tungsten nitride film and the tungsten film are formed on the titanium film. Hence, the gate electrode including the ohmic film of titanium or titanium nitride has a relatively high sheet resistance, so that the semiconductor device including the gate electrode may not have desired electrical characteristics.